There is often debate about how media in violence impacts children. I'd like to make my own little statement on the matter.
Violence means nothing outside context. Violent video games and movies do not in and of themselves cause violence; it is a question of how the violence is portrayed. A TV series like Band of Brothers doesn't promote war or shooting people for its own sake. It may not necessarily deter violence as a method to achieve a means, but that is an entirely different point in and of itself. For a comparison, I will use Call of Duty's single player story and multiplayer deathmatch modes to delineate how I feel violence should be examined in the context of ratings and censorship.
A quick disclaimer; I'm not very well versed on Call of Duty, and I'm writing more from the context of the first three games (the ones with real numbers) rather than World at War or Modern Warfare or Black Ops. In the first games the single player story was an attempt to recreate war for the player. While this isn't necessarily for everyone, the game tried to present both the thrilling action but also the horrible consequences for both sides. As such, I would say that Call of Duty's single-player elements were probably safe for a teenager to experience, since it provided a source of morals on war and violence in context. On the other hand, the deathmatch modes didn't focus so much on the horror of war so much as reflexes and shooting.
My point is this; violence for violence's sake is bad. I'm wholly against giving children the likes of Duke Nukem Forever or Borderlands in which the prime focus is violence (though there could be an argument made that Borderlands has more depth, the players and their characters tend to view violence with detached apathy. Indeed, I don't think I'd give even a game like Soldat in which the violence is more detached from reality on account of its form to a child. These games present a lack of value of humanity; their focus is on death and destruction rather than an improvement of society.
However, even in violent video games, it is not necessary for violence to be the only focus-there are many violent games, such as Fallout, where the primary focus is not the violence but its ends. Fallout is not one of the better examples, but to look to the games of my youth, MechWarrior provided an opportunity for me to see both violence as a destructive force and also as a way to exert change in the world. Above all, though, MechWarrior didn't pull punches, which is why I consider it to actually have been beneficial to my development (other than the basic math and analytical skills it required me to cultivate) through how it and many other "violent" games provide an opportunity for learning about social creeds and ideologies, as well as human interaction; even when I was playing at being a mercenary pilot I got to see how people had value, and I was operating in such a way as to reform society.
I don't deny a bias on this subject-I grew up playing violent games (though within moderation), write games that aren't built with pacifism in mind, and tend to be more pragmatic than idealistic when it comes to violence as a means to an end. That said, I do not dispute that violence in and of itself is morally negative (though it is often better than inaction).